WASHINGTON, Aug. 28 /PRNewswire-USNewswire/ -- Blood transfusions are a
valuable treatment mechanism in modern medicine, but can come with the risk
of donor disease transmission. Researchers are continually studying the
biology of blood products to understand how certain diseases are
transmitted in an effort to reduce this risk during blood transfusions.
According to a study in sheep prepublished online in Blood, the official
journal of the American Society of Hematology, the risk of transmitting
bovine spongiform encephalopathy (BSE, commonly known as "mad cow disease")
by blood transfusion is surprisingly high.

BSE is one of a group of rare neurodegenerative disorders called
transmissible spongiform encephalopathies (TSEs), and there is no reliable
non-invasive test for detecting infection before the onset of clinical
disease. In addition to BSE, these diseases include scrapie, a closely
related disease in sheep, and Creutzfeld-Jakob disease (CJD) in humans,
which causes neurological symptoms such as unsteadiness and involuntary
movements that develop as the illness progresses, rendering late-stage
sufferers completely immobile at the time of death.

A new variant of CJD (termed vCJD) was recognized in the United Kingdom
in the mid-1990s, apparently as a result of the transmission of BSE to
humans. Because the symptoms of this disease can take many years to appear,
it was not known how many people might have been infected, and without a
reliable test for identifying these individuals, clinicians were very
concerned that the infection could be transmitted between people by blood
transfusion or contaminated surgical and dental instruments. As a result,
costly control measures were introduced as a precautionary measure to
reduce the risk of disease transmission, although at the time it was
unclear whether there really was a significant risk or whether the control
measures would be effective. This sheep study sought to better understand
how readily TSEs could be transmitted by blood transfusion in order to help
develop more targeted controls.

"It is vitally important that we better understand the mechanisms of
disease transmission during blood transfusions so we can develop the most
effective control measures and minimize human-to-human infections," said
Dr. Fiona Houston, now a Faculty of Veterinary Medicine, University of
Glasgow, UK, and lead author of the study.

The nine-year study conducted at the University of Edinburgh compared
rates of disease transmission by examining blood transfusions from sheep
infected with BSE or scrapie; the BSE donors were experimentally infected,
while the scrapie donors had naturally acquired the disease. While scrapie
is not thought to transmit to humans, it was included as an infection
acquired under field conditions, which could possibly give different
results than those obtained from experimentally infected animals. Because
of the similarity in size of sheep and humans, the team was able to collect
and transfuse volumes of blood equivalent to those taken from human blood
donors.

The outcome of the experiment showed that both BSE and scrapie could be
effectively transmitted between sheep by blood transfusion. Importantly,
the team noted that transmission could occur when blood was collected from
donors before they developed signs of disease, but was more likely when
they were in the later stages of infection. Of the 22 sheep who received
infected blood from the BSE donor group, five showed signs of TSEs and
three others showed evidence of infection without clinical signs, yielding
an overall transmission rate of 36 percent. Of the 21 infected scrapie
recipients, nine developed clinical scrapie, yielding an overall
transmission rate of 43 percent.

Investigators noted that the results were consistent with what is known
about the four recorded cases of vCJD acquired by blood transfusion in
humans. In addition to the stage of infection in the donor, factors such as
genetic variation in disease susceptibility and the blood component
transfused may influence the transmission rate by transfusion in both sheep
and humans.

"The study shows that, for sheep infected with BSE or scrapie,
transmission rates via blood transfusion can be high, particularly when
donors are in the later stages of infection. This suggests that blood
transfusion represents an efficient route of transmission for these
diseases," said Dr. Houston. "Since the results are consistent with what we
know about human transmission, the work helps justify the control measures
put in place to safeguard human blood supplies. It also shows that blood
from BSE- and scrapie-infected sheep could be used effectively in non-human
experiments to answer important questions, such as which blood components
are most heavily infected, and to develop much-needed diagnostic tests."

The American Society of Hematology (http://www.hematology.org) is the
world's largest professional society concerned with the causes and
treatment of blood disorders. Its mission is to further the understanding,
diagnosis, treatment, and prevention of disorders affecting blood, bone
marrow, and the immunologic, hemostatic, and vascular systems, by promoting
research, clinical care, education, training, and advocacy in hematology.

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